Daiichi Sankyo Company, Limited announced the initiation of a three-part open-label phase 1/2 study in Japan with U3-1402, in patients with HER3-positive metastatic or unresectable breast cancer who are refractory or intolerant to standard treatment, or for whom no standard treatment is available. U3-1402, the second clinical-stage investigational antibody drug conjugate in the Daiichi Sankyo Cancer Enterprise pipeline, comprises a humanized anti-HER3 antibody attached by a peptide linker to a novel topoisomerase I inhibitor (DXd) payload, utilizing Daiichi Sankyo's proprietary payload and linker-payload technologies. U3-1402 achieves a Drug-Antibody Ratio (DAR) near 8, meaning nearly eight molecules of DXd are attached per antibody.  In this three-part open-label phase 1/2 study, U3-1402 will be given as an intravenous infusion every three weeks. The first part of the study (dose escalation) will assess the safety, tolerability and maximum tolerated dose of U3-1402 in HER3-positive metastatic breast cancer patients who are refractory or intolerant to standard treatment, or for whom no standard treatment is available. The second part of the study (dose-finding) will assess the safety and efficacy of U3-1402 and determine the recommended phase 2 dose in HER3-positive metastatic breast cancer patients who have received six or fewer prior chemotherapy regimens. The third part of the study (phase 2) will assess the safety and efficacy of the recommended dose of U3-1402 in HER3-positive metastatic breast cancer patients who have received six or fewer prior chemotherapy regimens.